An electric power steering apparatus (EPS) which is equipped with an electronic control unit and provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control. The motor is driven by the inverter which is constructed with the FET bridges.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a pinion-and-rack mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13 as a power supply, and an ignition key (IG) signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the calculated current command value. The steering angle θ can be obtained from a rotational sensor connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a micro controller unit (MCU) (including a central processing unit (CPU), a micro processor unit (MPU), a microcomputer and the like), and general functions performed by programs within the MCU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result I (=Irefm−Im) in the subtracting section 32B is proportional-integral-controlled (PI-controlled) at a PI-control section 35. The voltage control value Vref obtained by the PI-control at the PI-control section 35 and a modulation signal (a carrier) CF are inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The addition result is further added with a convergence control value 341 at an adding section 345. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the control characteristics.
In a case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 36 and the inverter 37 have a configuration as shown in FIG. 3, and the PWM-control section 36 comprises a duty calculating section 36A that calculates the duty signals D1 to D6 which are used in a three-phase PWM-control by using the voltage control command value Vref in accordance with a predetermined equation, and a gate driving section 36B that ON-drives or OFF-drives the FETs as a semiconductor switching device by means of the duty signals D1 to D6 and compensates a dead time. The modulation signal (the carrier) CF is inputted into the duty calculating section 36A, and the duty calculating section 36A calculates the duty signals D1 to D6 of the PWM by synchronized to the modulation signal CF.
The inverter 37 is configured to the three-phase bridges of the upper-stage FETs FET1 to FET3 and the lower-stage FETs FET4 to FET6. The motor 20 is driven by turning-ON or turning-OFF the FETs FET1 to FET6 by using the duty signals D1 to D6 of the PWM, respectively. The FETs FET1 to FET6 are the FET with a parasitic diode, respectively.
As well, a motor release switch 23 is interposed between the inverter 37 and the motor 20 in order to block a current supply for safety when the assist control is stopped and the like. The motor release switch 23 comprises the FETs with the parasitic diode disposed to the respective phases.
In such an inverter of the electric power steering apparatus, conventionally, in a case that the FETs FET1 to FET6 of the inverter 37 is a short failure, in order to prevent from that an overcurrent to the inverter 37 continuously flows, power supply relays (mechanical relays or semiconductor relays) 37B and 37C are disposed on power supply lines of a current detecting circuit 37A for detecting an inverter current of the inverter 37, the MCU and the inverter 37, as shown in FIG. 4. In an example shown in FIG. 4, although the inverter current is detected by a one-shunt type means, the inverter current may be detected by a two-shunt type means or a three-shunt type means.
The MCU diagnoses that the overcurrent flows to the inverter 37, in a case of detecting such a short failure, blocks the overcurrent by turning-OFF the power supply relays 37B and 37C and insures the safety of the system. For example, Japanese Unexamined Patent Publication No. H10-167085 A (Patent Document 1) discloses a protection circuit with respect to the inverter of a two-phase motor. However, there is a problem that the power supply relay for blocking the overcurrent is expensive, and it is not especially adequate that the power supply relay is used to the electric power steering apparatus of the vehicle which a cost reduction is strongly required.